1. Field of the Invention
The present invention relates generally to an energy absorber and more particularly to a wire bending load limiter device having utility wherever shock load restraint under various dynamic conditions may be required.
2. Description of the Prior Art
One problem which constantly recurs in the design and application of load carrying systems, such as cargo carring helicopters, is that of protecting and restraining the load when it is subject to dynamic shock loads such as those resulting from a hard landing or crash. If this is not done, the load can break loose adding significantly to the damage level incurred and presenting extra hazzards to flight personnel and passengers. To prevent this, a cargo restraing operating as a tension device is required to convert the crash pulse into a controlled and limited displacement of the cargo and maintain the restraint load level below the tie-down rupture point. In this field, significant requirements include the ability to accommodate combined loads and the constraints of geometry and human tolerances to `g` loading. For example, in seat installations the system kinematics, energy absorber type, and location greatly determine the effectiveness of the device used. In cargo application, additional significant general requirements which must be met are that it not impede use, servive all manner of use and abuse, and be compatible with all applicable environmental factors.
For any use, the energy absorption device should be used passively. That is, it should become so much a part of the restraint that there is no temptation not to not use it when cargo is tied down. Specific characteristics that would promote its use are small size and weight, and a shape that would aid rather than discourage use. In addition, rugged packaging is essential. Both the cargo tie-downs and the load limiter crash attenuator must survive severe usage encountered in handling, dropping, scraping, crushing imposed by wheeled vehicles, lack of stowage, use as a tow rope, and many other such non-anticipated functions. Coping with environmental factors include requirements that mud, sand, and corrosive ingredients must be excluded or be reduced to a non-effective role. Lastly, the load limiter must be able to accomodate wide varities of cargo configuration, tie-down direction, and crash pulse shapes with the capability of absorbing both overload and secondary impact conditions. The device should also be low in cost and essentially maintenance free. For such use, it is desirable that such an attentuator produce a constant rate of deformation at a constant force level. The force level should be constant within a close tolerance, from unit to unit, and remain constant as it is used.
Many energy attentuating principles have been demonstrated in the last ten years. These units have been employed in several ways, including simple tension (suspending the load), simple compression (supporting the load as a column), and combined tension and compression (providing rebound capability). In use they involve working of metal so as to limit loads by friction, torsion, shearing, crimping, compressing, expanding, or bending. In particular, devices employing wire bending have shown considerable promise, and units, such as those described by Reilly in U.S. Pat. No. 3,968,863; Jackson in U.S. Pat. No. 3,087,584; and Russo in U.S. Pat. No. 3,372,773, which are all based on this principle, have met with some degree of success. However, these units are usually made to withstand a particular load or units of gravity times a weight value on a static strength design basis. But, in dynamic shock situations typified by a relatively short, high amplitude tensile pulse, the instantaneous maximum load may easily exceed the nominal load carrying capabilities of the restraint system even though the total energy being absorbed is well within system design capabilities. When this happens, the cargo can break loose with resulting increased hazard to passengers and to the flight and, possibly, rescue crews.
A second problem has been that many of these methods have usually been expensive and/or difficult to apply in practice. In use, they have tended not to be too dependable and are particularly susceptible to weather and aging. In addition, the designs were such that they could not easily be adjusted to handle a range of design criteria. The subject invention is designed to correct these problems.